1. Field of the Invention
This invention relates in general to a system for tensioning a section of casing extending between a subsea well housing and a surface wellhead located on an offshore platform, and in particular to a system utilizing a rotatable running tool and an adjustable sub.
2. Description of the Prior Art
In one type of offshore drilling, particularly employed with jack-up drilling rigs, a wellhead housing will be located at the platform. A string of casing in the well will be supported in a subsea well housing, either by a mudline hanger or by a casing hanger. A string of casing extends from the subsea housing to the surface wellhead. Adjustment of the length of this casing string is required since the distance from the subsea well and the surface wellhead is not known, and the distance varies from well to well. Also, the string of casing between the subsea housing and surface wellhead needs to be tensioned.
Currently, there are three basic ways of providing adjustment and tensioning. One manner is to incorporate a removable load ring on the casing hanger. This load ring is usually threaded and rotated until it rests on the load shoulder of the surface wellhead high pressure housing. The second method entails the movement of the entire surface wellhead relative to the subsea housing. Each wellhead is adjusted and will be a different height relative to the platform floor. The third means of adjustment is to effectively change the length of the casing by having an adjustment sub within the casing string. The length of the casing is adjusted by various means so that the casing hanger will rest on the high pressure housing load shoulder. While workable, improvements are desired.